1. Field of Endeavor
The invention concerns the field of materials science. It relates to a special braze alloy for high-temperature brazing and to a method for repairing damaged components, in particular gas turbine blades or vanes, which are formed of single crystal or directionally solidified superalloys using the braze alloy. The invention also relates to a method for connecting at least two component elements made of single crystal or directionally solidified superalloys for the purpose of producing or repairing components using the braze alloy.
2. Brief Description of the Related Art
At very high loading temperatures, single crystal or directionally solidified components made of superalloys, for example nickel-based, cobalt-based, or nickel-cobalt-based superalloys, have, inter alia, good material strength but also good corrosion resistance and good oxidation resistance as well as good creep strength. On account of this combination of properties, the intake temperature of the turbine can be increased greatly when using such, nevertheless very expensive, materials, e.g., in gas turbines, and therefore the efficiency of the plant increases. Operating temperatures in the hot gas range of above 1400° C. are therefore loading temperatures to which a large number of components of the gas turbine, e.g., guide vanes and rotor blades or combustion chamber liners, are exposed. In addition to these high thermal stresses, turbine rotor blades in particular are also subjected to high mechanical loading, for example. During operation of the turbine, this as a whole can lead to the creation of undesirable cracks in the material, and therefore components damaged in this way either have to be replaced by new parts or else repaired.
Since, as already mentioned above, the production of single crystal or directionally solidified new turbine components is extremely expensive, however, and is complicated in the case of large parts in terms of a sufficient material quality (continuous single crystal or directionally solidified structure), an attempt is usually made to repair the damaged component present, i.e., the functionality of the component should be restored by the repair and the repaired component should then be used again for a further maintenance period in the turbine.
Compared to the repair of damaged components having a conventional polycrystalline microstructure, however, the repair of damaged single crystal or directionally solidified gas turbine components is significantly more difficult, since the repaired regions of the single crystal or directionally solidified components should also have a corresponding single crystal or directionally solidified microstructure; otherwise, the properties are undesirably impaired in the repaired region.
It is known prior art (see, e.g., EP 1 258 545 B1) to repair damaged gas turbine components by using a brazing process, for example. In this case, a braze alloy is applied to the base material in the region of the material damage to the component, e.g., in the region of a crack, and introduced into the crack, and is then melted by the action of heat (the treatment temperature has to be greater than the melting temperature of the braze alloy but less than the melting temperature of the base material) and integrally bonded to the base material. Melting point depressants, in the case of EP 1 258 545 B1 1-3% by weight B, are usually added to the braze alloy in order to reduce the melting temperature thereof.
Compared to the welding processes for repairing damaged gas turbine components, which are likewise known but not described in more detail here, the brazing process has the advantage that the base material is not melted during the brazing and therefore the single crystal structure of the base material can remain intact.
Diffusion processes occur in the material during the heat treatment in the case of brazing, and these have the effect, inter alia, that the melting point depressants, such as boron, diffuse from the braze alloy into the surrounding base material. The braze alloy solidifies as a consequence of the reduction in the boron concentration, whereas the base material has an increased boron concentration in the region surrounding the braze alloy, which can disadvantageously lead to the precipitation of brittle borides.
Furthermore, it is also disadvantageous that the brazing material, in contrast to the base material, in many cases cannot have a single crystal or directionally solidified structure after the brazing on account of the major action of heat. This can be attributed, among other things, to the fact that the high-temperature-resistant superalloys used for gas turbine components also have to be brazed at very high temperatures. Depending on the level of the residual stresses within the region to be repaired, for example a crack, the probability of recrystallization along the surface of the crack is then very high. This applies in particular to the surfaces which are subjected to machining, for example grinding, sandblasting or shot peening, during the preparation process, before the brazing cycle.
As a result of recrystallization, grains are newly formed in the base material, i.e., firstly a single crystal or directionally solidified structure can no longer be ensured in the base material and secondly the newly formed grain boundaries are not stable. The brazing material also solidifies in an unordered polycrystalline structure and therefore disadvantageously has poorer properties than the single crystal or directionally solidified base material.
A polycrystalline structure in the brazing material and recrystallization in the base material can only be prevented if the brazing temperature can be kept low enough below a critical value.
It is known from EP 1 759 806 A1 and from U.S. Patent App. Pub. No. 2004/0050913 A1 to reduce the melting point of a braze alloy by reducing the particle size (to values in the nanometer range) of the braze alloy, which is suspended in a carrier liquid, but this is done with the aim of reducing the proportion of melting point depressants, e.g., B and Si, in the braze alloy or of removing these depressants entirely from the braze alloy, since they are disadvantageously responsible for the formation of brittle phases, which, inter alia, cause an undesirable loss of ductility of the material.
The effect achieved by the use of braze alloy powder in the nanometer size range is therefore utilized here for replacing the melting point depressants in the material. The reduction in the melting point of the particles in the nanometer size range is explained by the low activation energy for releasing atoms on the surface of a particle in the nanometer size range as compared to a larger particle. In addition, nanoparticles melt faster than powder particles in the micrometer range, since they have a very large surface-to-volume ratio. This technical solution has the disadvantage that, on account of the sole use of nanoparticles as the solid braze alloy component of the suspension, strong shrinkage occurs after the brazing and therefore the quality of the brazed joint needs to be improved.
As a further possibility for additionally reducing the melting temperature of the nanoparticles when repairing single crystal components made of superalloys by brazing, EP 1 759 806 A1 also indicates that it is possible to add melting point depressants, in particular boron, directly to the braze alloy suspension.
U.S. Patent App. Pub. No. 2004/0050913 A1 additionally discloses a braze alloy material for diffusion brazing, which is formed of a powder mixture of filler material particles in the nanometer size range (preferably between 10 and 100 nm) and of powder particles in the micrometer size range (preferably between 45 and 100 μm) in a carrier suspension. As already mentioned above, the nanoparticles melt at a temperature which lies significantly below the melting temperature of particles having a particle size in the micrometer range, and therefore that document again makes express reference to the fact that it is thus advantageously possible for the addition of melting point depressants, such as B or Si, to the braze alloy to be reduced considerably or for the addition of melting point depressants to be dispensed with entirely, and therefore the negative effects which the melting point depressants have on the resulting properties of the brazed joint can be minimized or eliminated completely. By reducing the proportion of melting point depressants, the proportion of additional grain boundary stabilizing elements, such as B, C, Hf, Re, and Zr, in the braze alloy is additionally also reduced.
U.S. Patent App. Pub. No. 2004/0050913 A1 also describes that the surface of the nanoparticles of the braze alloy can optionally be coated with a very thin layer of melting point depressants, such as B or Si, although the overall proportion of melting point depressants in the braze alloy is still significantly lower compared to the proportion according to the known prior art, which is emphasized as being an advantage in that document.
Furthermore, EP 1 930 116 A2 discloses a method for repairing a metallic component having a crack. In this method, first a nanoparticle alloy in the form of a powder, a foil, a suspension or a paste is introduced into the crack, and a filler alloy which is at least similar to the base material and has a particle size in the micrometer range is applied thereover and then subjected to a conventional diffusion brazing process. The nanoparticles preferably are formed of an Ni-, Co- or NiCo-based alloy, which preferably additionally includes at least one metal from the group consisting of Ti, Cr, Nb, Hf, Ta, Mo, W, Al, and Fe. By using these materials, it is possible to repair large cracks at relatively low brazing temperatures, this document likewise stating that it is an advantage that the content of melting point depressants can be reduced and the mechanical properties of the metallic component are thereby retained. This technical solution has the disadvantage that, on account of the sole use of nanoparticles in the crack, strong shrinkage occurs after the brazing and therefore the quality of the brazed joint would appear to need improvement.
Finally, EP 1 967 313 A1 describes a braze alloy for repairing turbine components which likewise includes two powder components, wherein the first component is a powder having particle sizes in the micrometer range (0.7-100 μm) and the second component is a powder having particle sizes in the nanometer range (less than or equal to 500 nm). According to an embodiment variant, the first component of the braze alloy, i.e., the powder having a particle size in the micrometer range, which is preferably an alloy, includes a melting point depressant, to be precise in particular only one melting point depressant from the following group: C, B, Hf, Si, Zr, Ti, and Ta. That document provides no information relating to the quantitative proportion of the melting point depressant in the composition of the first powder. The braze alloy can be applied to or into the damaged site in the form of a paste, a slurry, in pure powder form, or as a foil. The difference between the melting temperature of the braze alloy and the melting temperature of the base material should be as high as possible, at least 70° C.
WO2008/095531 A1 describes a braze alloy composition and a brazing method for superalloys. The braze alloy composition does not include any melting point depressants, but instead is formed of a base material, preferably nickel (or else MCrAlX), and at least one initial phase, preferably aluminum. A twofold heat treatment is carried out, with the first heat treatment being carried out at a temperature at which the initial phase (relatively small Al particles) melts but the base material (Ni) still does not. The initial phase then completely surrounds the relatively large Ni particles. The second heat treatment is then carried out above a temperature at which at least one resulting phase, here nickel aluminide, forms, the solidus temperature of which is higher than the solidus temperature of the initial phase. If the resulting phase after the second heat treatment has mechanical properties which approximate the mechanical properties of the base material, it is possible to bring about reliable joining, e.g., closure of a crack. Here, it is therefore possible to use only strictly limited specific braze alloy compositions which additionally depend greatly on the Al content.